Switching devices for the switching of electric currents typically comprise at least one contact system and other housing elements. The contact system forms an electric switch and serves to switch electric currents. One class of switching devices are referred to as “circuit-breakers”, which can typically switch currents of 100 A and more. These circuit-breakers comprise a housing, in which the individual phases of the currents are switched. The individual phases of the currents can be located in pole cassettes, which are enclosed in their own housing. Located in the pole cassettes are moveable and fixed contacts, which can be mechanically separated or grouped together respectively, to switch the currents off and on respectively. At the separation of moveable and fixed contacts of a pole cassette, an arc occurs, which is typically extinguished in an “arcing chamber”. Circuit-breakers are likewise known which do not contain pole cassettes, and which accommodate movable and fixed contacts in their housings.
In circuit-breakers it is necessary, in order to achieve a good current limitation, to rapidly build up a high arc voltage. This is achieved with what are referred to as “double contact elements”, which subdivide the contact gap twice and so create two arcs simultaneously in the event of a short-circuit. The arc voltage produced by the arc is now available twice over in the same time unit, which improves the current limitation in comparison with simple circuit-breaking systems. Typically, with what are referred to as “double contact elements”, two electrical contacts are arranged on a rotatably-mounted contact bridge, which represent the moveable contacts. The two moveable contacts interact with two fixed contacts of the electric switch in order to close or open the circuit.
DE 692 09 972 T2 describes a circuit-breaker which includes single-pole units. With this circuit-breaker, the contact bridge of each pole cassette is mounted hanging freely in a switching shaft section, and the rigid mechanical connection between the individual switching shaft sections is effected by means of two rods arranged parallel to the switching shaft and eccentrically in relation to its axis of rotation. This arrangement guarantees the imposition of the contact force, the dynamic contact opening of the electric switch in the event of a short-circuit, and the coupling to a switch mechanism for opening and closing the electric switch with a handle.
In DE 693 04 374 T2 a circuit-breaker is disclosed with a press-molded housing with delay at the movement end of the contact bridge rejection. The contact bridge is mounted in the rotor housing without an axle. To achieve this, the contact bridge comprises tension springs, which in the switched-on position of the circuit-breaker, serve to guarantee that a pressure force is exerted by the contact bridge onto the fixed contacts, and at the same time allow for a rotation of the contact bridge under the effect of the electrodynamic forces in the direction of the rejection-open position.
Double-break contact systems with a rotational arrangement are very frequently prone to asymmetries. The reason for the asymmetries may lie in the tolerance position of the components, or, respectively, in the asymmetric erosion during operation. For example, the contact elements of the contact bridge may burn asymmetrically. These asymmetries lead to unequal contact forces and contact resistances at the contact points. Possible solutions known hitherto for avoiding these asymmetries make provision for a compensation possibility by the movable contact bridge, as well as by a floating arrangement of the switching shaft or, respectively, of the rotor in the pole cassette.